Acetylcholinesterase (AChE, EC3.1.1.7) is a key enzyme in neuronal signal transduction that hydrolyzes the neurotransmitter acetylcholine (ACh). The potential roles of AChEs involved in the toxicological and physiological impacts of insecticides on the destructive pest Spodoptera frugiperda, however, are still exclusive. In the present study, two acetylcholinesterase genes, ace1 and ace2, were characterized from S. frugiperda transcriptome and genome. Spatial−temporal expression analysis indicated that both Sfruace1 and Sfruace2 had an enriched expression in the heads among all larval tissues, and in 3rd instar larvae among all developmental stages. Notably, the expression levels of Sfruace1 were much higher than Sfruace2 in all tested tissues and developmental stages. The RNA interference (RNAi) with specific designed small interfering RNA (siRNA) significantly reduced the expression of Sfruace1 to 30%, and Sfruace2 to 39%. The knockdown of Sfruace1 expression resulted in mortality of 37.6%, which is significantly higher than 17.06% in the siSfruace2-treated group. Furthermore, the RNAi of Sfruace1 and Sfruace2 expressions reduced the AChE enzymatic activity to 32.81% and 65.77%, respectively, compared with those of the untreated group, 24 h after injection of 70 ng siRNA per insect. The survivors after the siRNA treatments showed an apparent motor retardation to the artificial diet, and an apparent growth inhibition in the parental generation (F₀) larvae. There was no significant inhibition effect on the growth of F₀ and the first filial generation (F₁) pupae and the fecundity of female adult. Insecticide bioassay showed that the siSfruace1-treated larvae were more susceptible to acephate than the siSfruace2-treated and untreated larvae. Our study suggests that Sfruace1 plays more important roles than Sfruace2 in larval survivorship and susceptibility to acephate, and both ace genes have differential roles in regulating larval growth, motor ability and insecticidal sensitivity in S. frugiperda.